1. Field of the Invention
The present invention relates generally to subsurface decontamination or remediation, and more particularly, to the separation of toxic metal ions, hydrophilic hydrocarbons and hydrophobic fuel and halogenated hydrocarbons from a primarily cationic flocculent, ethanol and water effluent stream.
2. Description of Related Art
The National Research Council (NRC) (1994) published a book entitled, "Alternatives to Groundwater Cleanup" that summarized the current state of the art in removing subsurface contaminants. In summary, the NRC arrived at the following conclusions. Current and developing technologies are cost-effective in remediating coarse-grained sediments (CGS) such as sands and gravels, but ineffective in removing contaminants from fine-grained sediments (FGS). The reason for the inability of present and developing technologies to remediate the FGS is primarily the very low hydraulic conductivity of FGS that effectively stops the flow of flushing fluids such as water, air and steam from penetrating the FGS and flushing contaminants away. In contrast, the CGS have hydraulic conductivities that are many orders of magnitude greater than that of FGS. Thus, advection can readily flush contaminants from such CGS.
In the copending, commonly assigned patent application, Ser. No. 08/495,294 entitled "A Nontoxic Chemical Process For In Situ Permeability Enhancement And Accelerated Decontamination Of Fine-Grain Subsurface Sediments", fully incorporated herein by reference, a method for remediating contaminated FGS is described in which safe cationic flocculents (CF) such as gypsum, ferric hydroxide, or aluminum polyhydroxide and organic solvents (OS), such as ethanol, are injected into both the subsurface saturated or groundwater zone and in the unsaturated or vadose zone to alter the FGS fabric to produce a flocculated and/or cracked material whose hydraulic conductivity is vastly increased. Both the CF and OS produce this desired result by collapsing the negative double layer surrounding the water saturated clay particles.
The organic solvent that should be considered in the subsurface remediation process should be chosen to have the following characteristics: It should not be prohibited by the United States Environmental Protection Agency (EPA) from being introduced to the subsurface, must be relatively inexpensive, must have a viscosity similar to water, must have a high boiling temperature and must be soluble in water. Furthermore, the following classes of chemicals should be either miscible or highly soluble in this organic solvent: hydrophilic hydrocarbons, and the hydrophobic fuel and halogenated hydrocarbons. This organic solvent should have a dielectric constant considerably lower than that of water. An example organic solvent that meets all the above characteristics is ethanol. The process in this invention is not limited to a specific organic solvent such as ethanol provided the replacement OS meets regulatory requirements and meets or exceeds these desired characteristics listed above.
The example organic solvent, ethanol has the distinct advantage in that sorbed hydrophobic contaminants readily partition into ethanol, the hydrophobic hydrocarbon contaminants are either completely miscible or highly soluble in ethanol, and hydrophilic contaminants are readily miscible in it.
In addition, clayey bodies that constitute FGS are intrinsically negatively charged due to the persistence of the negative double layer in water saturated clays. In the unaltered state, toxic metal ions such as the radioactive series as well as substances like As, Cd, Cr, Cu, Pb, Hg, etc., cations are attached to the clay bodies. Random perturbations in the ground water flow may dislodge such contaminants, providing a continuing source of pollution for many years, even though the high hydraulic conductivity soils have been remediated.
The alteration of the clayey fabric of the FGS due to the introduction of cationic flocculents (CF), a suitable OS like ethanol, and water-and OS soluble chelates will mobilize the following contaminants: toxic metal ions, hydrophobic fuel and halogenated hydrocarbons, and hydrophilic hydrocarbons that will be extracted from the subsurface.
The economic viability of the copending patent application referred to above, that deals specifically with remediating contaminated fine-grained sediments, depends highly upon contaminated bulk reduction, since the proper disposal of contaminants is very expensive. The bulk reduction is accomplished by a special combination of existing technologies to achieve the stated goal of contaminant bulk reduction.
In current technologies dealing with the remediation of contaminated subsurfaces, the following processes are typically observed in treating the contaminant effluent stream:
1. Removal of soil particles form the effluent stream.
2. The removal of hydrocarbons by a combination of processes:
A. Air-stripping the readily volatilized hydrocarbons, and PA1 B. Adsorption of contaminants onto an activated carbon bed.
3. Ultra-violet light destruction of halogenated hydrocarbons.
4. Purification of the groundwater for reinjection.
5. Purification of the vapor phase stream.